As will be appreciated by those skilled in the art, helical shaped devices include such items as the common screw and the like. In addition, specialized helical shaped devices such as screw conveyors are well known for the transport of various types of bulk material. Such conveyors generally include a helical shaped member which rotates within a housing or trough such that rotation of the screw conveyor along its longitudinal or central axis results in movement of the bulk material along the length of the device. In the past, such conveyors were typically constructed from metal, and each conveyor was specifically designed for a particular purpose and of the necessary length. However, although most prior art conveyors were specifically designed to meet a particular application, there have been some attempts at providing a modular type conveyor of metal and wood. Examples of U.S. patents which disclose such modular conveyors include U.S. Pat. No. 349,233 issued to James Nelson on Sept. 10, 1886; U.S. Pat. No. 455,384 issued to H. Binkholz on July 7, 1891; U.S. Pat. No. 525,194 issued to J. Dyson, et al on Aug. 20, 1894; U.S. Pat. No. 546,879 issued to J. Dyson, et al on Sept. 24, 1895, U.S. Pat. No. 1,867,573 issued to V. G. Leach on July 19, 1932; U.S. Pat. No. 2,492,915 issued to A. B. Carlson on Dec. 27, 1949; and U.S. Pat. No. 3,648,826 issued to Dean P. Brooks on Mar. 14, 1972. However, a study of the above patents indicates that perhaps only the two Dyson, et al patents and the Nelson patent can be considered sectional or modular. Consequently, the inventor of the invention covered by the present application filed an application for a United States letters patent on Sept. 29, 1983 for a "Modular Screw Conveyor" and which has U.S. Ser. No. 537,345.
To date, the applicant is not aware of any type of helical shaped screw conveyor formed by molding, since the molds for such devices have in the past been considered expensive complex and difficult to operate. Further, until this invention, helical modules having a one-piece hollow axial shaft as an integral part of its molded product were thought to be unmoldable in a simple two piece mold without side actions. For example, U.S. Pat. No. 3,891,367 issued to Maurizio Signora on June 24, 1975 discloses a process for forming helical gears from a metal powder by compression. According to this patent, a portion of the die or mold for forming the helical gear is required to undergo helical motion as it moves axially to open and close the mold. Absent such rotating or helical motion, the formed material would prevent axial opening of the two pieces of the die or mold. It will be appreciated of course that a requirement of helical motion by a portion of the die greatly increases the complexity and cost of the molding process. Other molds in prior use for molding helical shaped devices include side opening molds. That is, molds which open perpendicularly to the axial center of the helical shaped mold rather than parallel or along the axial center. However, because of the helical shape of the flight member it is necessary that a flight member formed by such side opening molds be molded with a substantially broader base than tip if the mold is to open at all. Consequently, although a side opening mold may be suitable for certain screw type fastening devices it is completely unacceptable for efficient screw type or helical shaped conveying system, since such conveying systems preferably have a flight members wherein the base is not substantially greater than the tip.
Attempts to mold other plastic devices such as single threaded nuts or wing-nuts, have resulted in molds which include portions which do move axial with respect to each other without rotational movement. For example U.S. Pat. No. 4,079,475 issued to Thompson on Mar. 21, 1978 discloses a mold for forming a fastening nut with a single internal thread of less than 360 degrees wherein the mold does open along the axial center of the mold without concurrent rotation. In a similar manner, U.S. Pat. No. 4,188,178 issued to Bernard Anscher on Feb. 12, 1980 discloses a mold for forming wing-nuts having a single internal thread and which requires the use of two forming steels for forming the tabs of the wing-nut and two piston portions which move axially with respect to each other for forming the internal portion of the wing-nut and the single full thread. However, an examination of these patent reveal that the molds are much more complex, and the resulting product has internal threads rather than an external thread. Furthermore, both the Auscher and Thompson patents require a four piece mold for operation.
From initial appearances, perhaps U.S. Pat. No. 546,879 as listed above would seem to show a casting mold which opens along its center axis for forming metal screw or helical shaped surfaces. However, a reading of the patent and study of the drawings clearly illustrates that it does require an initial rotation of one of the portions of the mold with respect to the other if the mold is to be separated along the center axis. Therefore of course, if the center portion of the mold which forms the axial member of the casting was not of a circular outside shape, such rotation would be impossible. In the Dyson,et al, patent however, such rotation is possible since the mold does not form an axial or center member at all, and only forms the helical portion or the flight members.
Other patents which illustrate the present state of the art for molding helical shaped or screw devices, include U.S. Pat. No. 3,461,949 issued to N. E. R. Nilson on Aug. 19, 1969; U.S. Pat. No. 3,664,212 issued to Hehl on May 23, 1972; U.S. Pat. No. 2,864,128 issued to Heisler et al on Dec. 16, 1958; U.S. Pat. No. 3,584,092 issued to Alexander on June 8, 1971; U.S. Pat. No. 3,905,416 issued to Hammer on Sept. 16, 1975 U.S. Pat. No. 4,059,249 issued to Galer on Nov. 22, 1977; and U.S. Pat. No. 4,139,176 issued to Wundsch on Feb. 13, 1979.
Thus a review of the prior art patents reveals that to date there has not been a simple and inexpensive mold and process for forming a helical shaped device having an axial member and certainly not such a mold and process suitable for injection molding, and most certainly not one having an axial member with a non-circular aperture molded therein. Therefore, it is an object of this invention to provide a two-piece mold and related molding process for forming, by injection molding, a helical shaped device which includes an axial member integrally joined with a helical shaped flight member.
It is another object of the present invention to provide a mold and process for forming helical shaped flight members integrally formed with an axial member which defines a non-circular aperture therethrough.
It is also an object of the present invention to provide a mold and molding process for forming a helical shaped device wherein the flight member extends radially from, and around an axial member and encloses up to 360 degrees.
It is still a further object of the present invention to provide a mold and molding process for forming a helical shaped device which may be joined axially with similar helical shaped devices for forming a screw conveyor of any desired length wherein the flight member extends around the axial member greater than 360 degree.
Other objects and advantages will be obvious, and will in part appear hereinafter, and will be accomplished by the present invention which provides a reusable mold and molding process for integrally forming a helical shaped device from a selected material. The formed helical shaped device includes an axial member integrally joined with one or more helical shaped flight member each of which has front and back surfaces. The flight member or members extend a selected distance radially from the axial member and in combination enclose up to 360 degrees coaxially around the axial member. The reusable mold operates or opens along a center axis and comprises a first section which partially defines an elongated cavity portion which extends along and around the center axis. This first section forms a portion of the axial member of the helical shaped device. The section further defines a helical surface which is also disposed coaxialy around and along the center axis and extends the selected distance radially from the axis and for the selected amount of rotation of no greater than 360 degrees. This first section forms one of the front and back surfaces of the helical shaped flight member. A second section similar to the first section, which is adapted for reciprocating movement along the center axis between an open and close position with respect to the first section also defines an elongated cavity portion which extends along the center axis. The second section of the mold also includes a helical surface similar to that of the first section which is disposed coaxially around and along the center axis. This second helical surface also extends radially from the axis for the same distance and for the amount of rotation as the helical surface on the first section such that it forms the other surface of the helical shaped flight member. The two sections of the mold cooperate such that when they are in a closed position, the helical surfaces of each section are substantially parallel to each other and are maintained at a selected and constant spacing so that in combination, the sections define a substantially enclosed cavity having an elongated axial portion and a helical shaped flight portion of a selected cross- section which extends radially from the center axis and is with the center axis for the selected amount of rotation.
In addition, at least one of the first and second sections define a sprue, gate or port which extends from the enclosed cavity to the outside of the mold such that molding material may be transferred to the inside of the mold to fill the cavity and integrally form the helical shaped flight member and axial member. In operation, the two sections open and close by axial movement with respect to each other such that the helical shaped device formed in the cavity may be removed by moving at least one of the sections along the center axis to the open position.
In a preferred embodiment, the mold further encloses a central rod member which is positioned coaxially , with the center axis and has a selected cross-section which is smaller than the cross-section of the elongated cavity in the mold. Typically, the central rod member extends the length of the elongated cavity such that when the molded item is removed from the mold, an aperture extends coaxially completely through, and coaxially with the axial shaped member. However, for some processes, such as metal castings for example, it may be desirable that an aperture extend only partway through the axial member. In such cases the central rod member will not extend the full length of the elongated cavity.
Also in the preferred embodiment, the defined aperture through the axial shaped member is non-circular and may preferably be square or hexagonal in shape.
The present mold and molding technique may also be used to form a helical shaped device which includes more than a single flight member. In the event more than one flight member is to be formed along and coaxial with the center axis, each of the helical shaped flight members will enclose a selected amount of the center axis for a number of degrees determined by dividing the selected number of flight members into 360 degrees. Further, each of such flight members will be located in a rotational position which is different from every other one of the flight members, such that none of the flight members overlap. For example it will be appreciated that if two flight members are used, each of the flight members will enclose substantially 180 degrees. Similarly, if three flight members are used each of the flight members will enclose substantially 120 degrees of the axial member.
It should also be understood that a multiplicity of the devices formed by the molding process of this invention may be axially joined to produce an elongated screw conveyor or helical shaped device of any desired or selected length which has at least one continuous helical flight member which extends around the center axis greater than 360 degrees.